1. Technical Field
The present invention relates to an RF cavity filter, more particularly to a cavity filter that uses cross-coupling.
2. Description of the Related Art
A filter is a device that allows only signals of a specific frequency band to pass through. Depending on the band-pass property, a filter may be classified as a low-pass filter, a band-pass filter, a high-pass filter, a band-stop filter, etc. Also, depending on the structure of the filter, a filter may be classified as a lumped circuit filter, a ceramic filter, a cavity filter, etc.
A filter may filter signals of a particular frequency by using resonance obtained by a combination of inductances and capacitances, and the band-pass properties may be determined based on the manner in which the inductances and capacitances are connected.
Two important characteristics are associated with the filter; one is insertion loss, and the other is its skirt property. Insertion loss refers to how much of the total inputted power is lost, while the skirt property refers to how steep the band-pass property curve is. The insertion loss and the skirt property are mainly related to the order of the filter, and form a trade-off relationship with each other, as a higher order of the filter would result in better skirt property but worse insertion loss.
A base station of a mobile communication system frequently uses the cavity filter, in which multiple cavities are formed and a resonator is held in each of the cavities to provide the required band-pass property and delay.
With technological advances in the field of RF filters, there is a growing demand from base station service providers for filters having compact sizes. There are various approaches towards implementing smaller filters. In the case of the macrocell filter, there is active research under way focusing on the TM mode filter, which is relatively easy to implement in a thinner form. In the case of the small cell filter, providing a compact size may be a more important factor, and small coaxial resonators utilizing a ceramic material can be used.
A similarity between these two types of technology is in the resonance mode. At the head portion of a coaxial resonator employing a ceramic material, the E-field includes a longitudinal component that is stronger than the transverse component, in contrast to the existing TEM mode, so that a Quasi-TM mode field distribution is obtained that is similar to a TM mode.
Generally, controlling the transmission zero is critically important in improving the attenuation properties of a BPF, and this is implemented by applying cross-coupling between non-adjacent resonators. While inductive cross-coupling can be implemented with the use of an aperture or window between two resonators, capacitive cross-coupling may require an additional structure such as a coupling member to implement negative coupling. A coupling member refers to a structure that provides gap coupling for the transverse direction E-field of the two resonators to increase the capacitance value.
However, in the case of the TM mode for TM mode resonators and in the case of the Quasi-TM mode for resonators employing a ceramic material, the longitudinal direction E-field component is stronger than the transverse direction E-field component, so that there is a limit to implementing capacitive cross-coupling by using a coupling member that requires a transverse direction E-field component.